1. Field of the Invention
The present invention relates to an exhaust gas purification device of an engine.
2. Description of the Related Art
In the related art, in a diesel engine, particulate contained in the exhaust gas is removed by arranging a particulate filter in the engine exhaust passage, using that particulate filter to trap the particulate in the exhaust gas, and igniting and burning the particulate trapped on the particulate filter to recycle the particulate filter. The particulate trapped on the particulate filter, however, does not ignite unless the temperature becomes a high one of at least about 600xc2x0 C. As opposed to this, the temperature of the exhaust gas of a diesel engine is normally considerably lower than 600xc2x0 C. Therefore, it is difficult to use the heat of the exhaust gas to cause the particulate trapped on the particulate filter to ignite. To use the heat of the exhaust gas to cause the particulate trapped on the particulate filter to ignite, it is necessary to lower the ignition temperature of the particulate.
It has been known in the related art, however, that the ignition temperature of particulate can be reduced if carrying a catalyst on the particulate filter. Therefore, known in the art are various particulate filters carrying catalysts for reducing the ignition temperature of the particulate.
For example, Japanese Examined Patent Publication (Kokoku) No. 7-106290 discloses a particulate filter comprising a particulate filter carrying a mixture of a platinum group metal and an alkali earth metal oxide. In this particulate filter, the particulate is ignited by a relatively low temperature of about 350xc2x0 C. to 400xc2x0 C., then is continuously burned.
Summarizing the problem to be solved by the invention, in a diesel, engine, when the load becomes high, the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., therefore with the above particulate filter, it would appear at first glance that the particulate could be made to ignite and burn by the heat of the exhaust gas when the engine load becomes high. In fact, however, even if the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., sometimes the particulate will not ignite. Further, even if the particulate ignites, only some of the particulate will burn and a large amount of the particulate will remain unburned.
That is, when the amount of the particulate contained in the exhaust gas is small, the amount of the particulate deposited on the particulate filter is small. At this time, if the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., the particulate on the particulate filter ignites and then is continuously burned.
If the amount of the particulate contained in the exhaust gas becomes larger, however, before the particulate deposited on the particulate filter completely burns, other particulate will deposit on that particulate. As a result, the particulate deposits in layers on the particulate filter. If the particulate deposits in layers on the particulate filter in this way, the part of the particulate easily contacting the oxygen will be burned, but the remaining particulate hard to contact the oxygen will not burn and therefore a large amount of particulate will remain unburned. Therefore, if the amount of particulate contained in the exhaust gas becomes larger, a large amount of particulate continues to deposit on the particulate filter.
On the other hand, if a large amount of particulate is deposited on the particulate filter, the deposited particulate gradually becomes harder to ignite and burn. It probably becomes harder to burn in this way because the carbon in the particulate changes to the hard-to-burn graphite etc. while depositing. In fact, if a large amount of particulate continues to deposit on the particulate filter, the deposited particulate will not ignite at a low temperature of 350xc2x0 C. to 400xc2x0 C. A high temperature of over 600xc2x0 C. is required for causing ignition of the deposited particulate. In a diesel engine, however, the temperature of the exhaust gas usually never becomes a high temperature of over 600xc2x0 C. Therefore, if a large amount of particulate continues to deposit on the particulate filter, it is difficult to cause ignition of the deposited particulate by the heat of the exhaust gas.
On the other hand, at this time, if it were possible to make the temperature of the exhaust gas a high temperature of over 600xc2x0 C., the deposited particulate would be ignited, but another problem would occur in this case. That is, in this case, if the deposited particulate were made to ignite, it would burn while generating a luminous flame. At this time, the temperature of the particulate filter would be maintained at over 800xc2x0 C. for a long time until the deposited particulate finished being burned. If the particulate filter is exposed to a high temperature of over 800xc2x0 C. for a long time in this way, however, the particulate filter will deteriorate quickly and therefore the problem will arise of the particulate filter having to be replaced with a new filter early.
Further, if the deposited particulate is burned, the ash will condense and form large masses. These masses of ash clog the fine holes of the particulate filter. The number of the clogged fine holes gradually increases along with the elapse of time and therefore the pressure loss of the flow of exhaust gas in the particulate filter gradually becomes larger. If the pressure loss of the flow of exhaust gas becomes larger, the output of the engine falls and therefore due to this as well a problem arises that the particulate filter has to be replaced quickly with a new filter.
If a large amount of particulate deposits once in layers in this way, various problems arise as explained above. Therefore, it is necessary to prevent a large amount of particulate from depositing in layers while considering the balance between the amount of particulate contained in the exhaust gas and the amount of particulate able to be burned on the particulate filter. With the particulate filter disclosed in the above publication, however, no consideration is given at all to the balance between the amount of particulate contained in the exhaust gas and the amount of particulate able to be burned on the particulate filter and therefore various problems arise as explained above.
Further, with the particulate filter disclosed in the above publication, if the temperature of the exhaust gas falls below 350xc2x0 C., the particulate will not ignite and therefore the particulate will deposit on the particulate filter. In this case, if the amount of deposition is small, when the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C., the deposited particulate will be burned, but if a large amount of particulate deposits in layers, the deposited particulate will not ignite when the temperature of the exhaust gas reaches from 350xc2x0 C. to 400xc2x0 C. Even if it does ignite, part of the particulate will not burn, so will remain unburned.
In this case, if the temperature of the exhaust gas is raised before the large amount of particulate deposits in layers, it is possible to make the deposited particulate burn without leaving any, but with the particulate filter disclosed in the above publication, this is not considered at all. Therefore, when a large amount of particulate deposits in layers, so far as the temperature of the exhaust gas is not raised to over 600xc2x0 C., all of the deposited particulate cannot be made to burn.
Further, known is an NOx absorbent which absorbs the NOx exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and which releases and reduces the: absorbed NOx when the air-fuel ratio of the inflowing exhaust gas becomes the stoichiometric air-fuel ratio or rich. If this NOx absorbent is arranged in the engine exhaust passage, the NOx generated during combustion under a lean air-fuel ratio can be made to be absorbed in the NOx absorbent.
The absorption action of the NOx by this NOx absorbent, however, only occurs in a certain temperature range determined by the NOx absorbent. The same applies even when giving an NOx absorption function to a particulate filter. Therefore, when giving an NOx absorption function to the particulate filter, it is necessary to keep a large amount of particulate from being deposited on the particulate filter and simultaneously maintain the temperature of the particulate filter in a temperature range where the NOx absorption action occurs.
An object of the present invention is to provide an exhaust gas purification apparatus designed to simultaneously remove the particulate and NOx in an exhaust gas by a novel method.
According to the present invention, there is provided an exhaust gas purification apparatus of an engine having a combustion chamber comprising an exhaust passage and a particulate filter arranged in the exhaust passage for removing particulate in exhaust gas exhausted from the combustion chamber; the particulate filter being a particulate filter having a function of removing, by oxidation, particulate in the exhaust gas without emitting a luminous flame when an amount of discharged particulate discharged from the combustion chamber per unit time is smaller than an amount of particulate removable by oxidation per unit time without emitting a luminous flame on the particulate filter and of absorbing NOx in exhaust gas when an air-fuel ratio of the exhaust gas flowing into the particulate filter is lean and releasing the absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the particulate filter becomes the stoichiometric air-fuel ratio or rich; the amount of the discharged particulate and the temperature of the particulate filter being usually maintained within a simultaneous particulate and NOx treatment region in a temperature region wherein the amount of discharged particulate becomes smaller than an amount of particulate removable by oxidation and a NOx absorption rate becomes more than a certain value at all times.